Logarithmic Scale
The luminosities of six dormant X-ray novas containing black
holes, as measured by Chandra, are compared to the luminosities
of six dormant X-ray novas containing neutron stars. The black
holes are represented as filled circles and the neutron stars as
open circles. The vertical axis is the logarithm of the dormant
luminosity in Eddington scaled units (which are the natural
units to use for the luminosity). The arrow indicates that one
black hole system has not yet been detected, but is fainter than
the point indicated on the graph. The horizontal axis gives the
orbital period of the binary systems. The amount of energy and
matter being accreted onto the neutron star or black hole
increases with increasing orbital period, so we expect those
systems with the longest periods to be the most luminous. For
this same reason, in order to make an unbiased comparison of the
luminosities of black holes and neutron star systems, one should
compare systems with similar orbital periods. The non-shaded
area highlights a region on the graph where the black hole and
neutron star systems have similar orbital periods.

Within this region, it is clear that the black hole systems are
100 to 1000 times fainter than the neutron star systems. Given
that the amount of energy and matter being accreted is similar
in both cases, the much lower luminosity of the black hole
systems indicates that they are 'hiding' the accretion energy
behind an event horizon.(Credit: NASA/CfA/M. Garcia et al.)

Linear Scale
The objects within the non-shaded area on the logarithmic plot
are shown here on a linear scale. Here we see luminosities of
four dormant X-ray nova containing black holes, as measured by
Chandra, compared to the luminosities of five dormant X-ray nova
containing neutron stars. The black holes are represented as
filled circles, and the neutron stars as open circles. The
vertical axis is the luminosity in Eddington units, scaled so
that the luminosity of the black holes is about 1 (Eddington
units are the natural units to use for the luminosity). The
horizontal axis gives the orbital period of the binary systems.
the arrows indicate that the luminosity is lower (for one black
hole system) or higher (for four neutron star systems) than
their respective points indicated on the graph.

It is clear from this graph that the black hole systems are at
least 100 times fainter than the neutron star systems. Given
that the amount of energy and matter being accreted is similar
in both cases, the much lower luminosity of the black hole
systems indicates that they have an event horizon which hides
more than 99% of their accretion luminosity.(Credit: NASA/CfA/M. Garcia et al.)